Recently, with the continual increment of the oil consumption and the improvement of the people's environmental consciousness, the demand on the oil consumption and smoke degree of the internal combustion engines using oil as an energy source is rigorous increasingly, and in order to improve oil quality and reduce the discharge of hazardous gases, a method of adding additives into the oil is relatively simple, economical and easy to be carried out effectively. The cetane number (CN) of diesel oil is an important index for scaling the performance of diesel oil. By increasing the cetane number, the discharge of the black smoke can be restrained effectively. Moreover, the self-oxygen feeding ability of the oxygen-containing additives has obvious effect during accelerating the combustion of oils. Polymethoxymethylal is a new clean oil additive and is an effective additive for improving the combustion of diesel oils, increasing the cetane number, reducing the discharge of carbon dioxide and NOx (nitrogen oxides), reducing the oil consumption, as well as reducing the smoke discharge. Polymethoxymethylal (RO(CH2O)mR, having a very high CN value and oxygen content (42-49% for the methyl series and 30-43% for the ethyl series)) can be added into diesel oil by 10%˜20% and reduces the discharge of NOx and CO substantially. Due to their vapor pressure, boiling point and solubility in oils, the polymethoxymethylal (DMM3-8) of 3≦m≦8 is generally appropriate for oil addition.
DMM3-8 is early obtained by using methanol, formaldehyde, poly formaldehyde or glycol formal as raw materials under the catalysis of sulfuric acid or hydrochloric acid. Recently, the research on synthesizing DMM3-8 with catalysis of liquid acids in the representative of BASF Co. has obtained a certain development (WO 2006/045506 A1). The series products with m=1˜10 were obtained by using sulfuric acid and trifluoromethylsulfonic acid as catalysts and using methanol, methylal, polyformaldehyde and trioxymethylene as raw materials. However, the catalysts are corrosive, the distribution of the products is not reasonable, and the conversion and the components available for oil additives are low, wherein, methylal is 33.5%, DMM2 is 23.6%, DMM3-11 is 28.3%, and the other m>11, wherein, the DMM3-8 as the effective additive component is below 28.3%. A multiphase catalysis has been developed in the representative of BP Co. (U.S. Pat. Nos. 6,160,174, 62,655,284) and obtained polymethoxymethylal by a multistep process using methanol, formaldehyde, dimethyl ether and methylal as raw materials. Wherein, methylal is retained by 22.6%, DMM2 is 12.42%, DMM3-11 is only 11.6% and the other m>11, therefore, the breakthrough in key technology has not been achieved.
Up till now, due to the disadvantages of relatively low activity, rigorous reaction conditions, complex process and the like for DMM3-8 synthesis catalysts, this technology has not been actually employed.